A while back Bill Layer described how to update the Cisco 675/678
bios with tftp.  He said he was not able to upgrade with minicom
and xmodem.

The trick to get xmodem to work from minicom is to set 1k mode.
Do this by going to the minicom cOnfigure menu (control-A O) and
selecting the File Transfer Protocols item.  Then for xmodem upload
(the sx program) change the Program field by adding the option "--1k".
On my system the full program field is then "/usr/bin/sx -vv --1k".
Don't change any other fields on the File Transfer Protocols screen
(the user interface on this screen is a little tricky/obscure).

Then you can upload the new CBOS firmware:
 
  - connect management cable
  - run minicom, 38400 8N1, no flow control on the serial port with the
management
    cable attached
  - press return in minicom to get login message from the Cisco 675/678
  - login to the Cisco 675/678
  - enter the "enable" command and the password
  - enter the "set download code" command
  - start the xmodem transfer from minicom
  - when the transfer is complete, reboot the Cisco 675/678

A tech at visi said xmodem was safer than tftp, since if the upload is
interrupted you are less likely to end up with a partially updated and
useless modem.  Don't know if that is true or not.

-- Al